1. Field of the Invention
The present invention relates to an electrolyte containing an electrolytic solution and a high molecular weight compound, and a battery using it.
2. Description of the Related Art
In recent years, many portable electronic devices such as a mobile phone and a laptop computer have been introduced. Downsizing and weight saving of these devices have been made. Along with these situations, as a portable power source for these electronic devices, a lithium ion secondary battery has attracted attention. Specially, a lithium ion secondary battery, wherein a gelatinous electrolyte in which an electrolytic solution is held or diffused in a high molecular weight compound is used has attracted attention as a next-generation battery because of the following reason. That is, there is no danger of leak, and a film exterior member can be used. Therefore, the battery, whose weight and thickness can be decreased, and whose freedom of shape is high can be realized.
In order to improve ion conductivity of the lithium ion secondary battery at low temperatures, solvents such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate are very effective, since they have high solidifying points and low viscosity. Therefore, in a lithium ion secondary battery, wherein only the electrolytic solution is used as an electrolyte, ethylene carbonate which is electrochemically stable and has a high dielectric constant and so on are used as a main solvent, and a low viscosity solvent such as dimethyl carbonate is mixed with such a main solvent. A battery capacity, cycle characteristics, load characteristics, and low temperature characteristics are thereby improved.
However, the solvent such as dimethyl carbonate cannot be used in large quantity for the secondary battery wherein the gelatinous electrolyte is used, particularly for the secondary battery wherein the film exterior member is used. The solvent such as dimethyl carbonate has a low compatibility with high molecular weight compounds, and has a low boiling point. Therefore, when using the solvent such as dimethyl carbonate, an inner pressure of the battery might be raised, and swollenness of the battery might be caused. Consequently, in the secondary battery using the film exterior member, a mixed solvent in which ethylene carbonate and propylene carbonate are mixed is widely used (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-167797 and Japanese Unexamined Patent Application Publication No. 2001-155790).
When using such a mixed solvent, low temperature characteristics and load characteristics can be improved by, for example, raising a mixing rate of propylene carbonate. However, in this case, an initial charge and discharge efficiency is lowered, resulting in extreme lowering of the battery capacity. Further, cycle characteristics tend to deteriorate as well. As above, the conventional gelatinous electrolyte has a problem that excellent battery capacity, cycle characteristics, load characteristics, and low temperature characteristics cannot be obtained at once.